Calorific Value For Natural Gas & MFO

Here's something that may help:

http://www.hrt.msu.edu/energy/pdf/heating%20value%20of%20common%20fuels.pdf

Burner installer told me natural gas burning is less efficient (less heat output) though overall cost is still cheaper. Is it true? yes. It is the price of natural gas rangeing 35% to 60% less which makes it cheaper.

Hi tofumaster,

As with all things it depends.

Was the conversion done by adding a gas kit to your oil burner or a complete replacement? If a kit was used then it should be possible to set the burner to achieve the same combustion efficiency or possibly, depending on the burner model, the air/gas ratio control fitted and just as importantly the skill of the commissioning engineer. It is not possible to comment on the effect of fitting a new burner without much more information. Did your contractor do a before and after combustion efficiency check? That would answer your efficiency question.

The CV of NG is slightly lower on a mass basis, but all that means is the burner has to pass slightly more gas to compensate and achieve the same firing rate. The products of combustion of NG contain more water vapour than MFO so the flue loss tends to be slightly higher at normal boiler flue exit temperatures as a result, all other things being equal.

The effect on the overall plant efficiency of changing fuels depends very heavily on the boiler. Water tube boilers depend very heavily on radiant heat exchange and this is adversely affected by the low emissivity of the NG flame against that of an oil flame. Flame tube boilers are still affected by the emmissivity differences but to a lower extent. The difference can cause problems in the reversal chamber with elevated tube end temperatures unless precautions are taken (ceramic ferrules or J weld prep). Overall it would be reasonable to expect on a new clean boiler to achieve very similar efficiencies with the oil slightly better (1-2%age points). However the heat exchanger surfaces will become fouled when firing on oil and will not on NG so after a time the efficiencies will equalise.

We did a coal to NG conversion years ago of a set of La Monte water tube boilers and even after they had been chemically descaled the efficiency improved over the following 12 months as the tubes continued to be cleaned by the hot gases so its not always easy to tell in advance.

Regards,

Gasman

It's a complete replacement...the new burner is a duel fuel burner....firing either mfo or gas...boiler is fire tube type

my concern here is since burner need to burn more gas compare to mfo to achieve same firing rate as mfo can we say this is the normal case...

Hi tofumaster.

Your dual fuel installation is set it seems to burn gas primarily with MFO as the secondary fuel. Therefore your burner engineer should have set the air/fuel rate to give the rated output as defined by the boiler manufacturer. Thereafter when oil is selected the amount of oil introduced is usually set to consume the available oxygen in the air supply. It is usually done this way as the air flow rate cannot be varied easily or automatically unless you have a very sophisticated burner set-up.

You will also find that the CO2 and excess air conditions different for the two fuels with a much higher Co2 for oil.

You may also wish to think about your tube end temperature as this tends to increase when firing gas and may in turn lead to erosion/cracking over time. You can prevent this by fitting ceramis ferrules to the mouth of the tubes. Having experienced this problem some years ago when we converted an oil fired boiler to natural gas (10MBTU flame reversal HTHW blr) I would advise caution.

Hope this helps,

Massey.

Your installer was correct in saying Cv is less than MFO. I'm not 100% sure of MFO but i think MFO Cv is around 44000 Kj/Kg whilst the CV for natural gas is 43000kj/kg. Although it is close to MFO you will use more natural gas to deliver the same efficiency/performance/heat tham MFO. Irespective of what a poster said, no amount of setting up, fine tuning will change the CV of your fuel. Natural gas is cheaper and therefor at the end of the day you will pay less for the same energy.

Your installer was correct in saying Cv is less than MFO. I'm not 100% sure of MFO but i think MFO Cv is around 44000 kJ/Kg whilst the CV for natural gas is 43000kJ/kg. Although it is close to MFO you will use more natural gas to deliver the same efficiency/performance/heat them MFO.

I have to take issue with your statement above: It is only true if you delete efficiency from the last sentence. Efficiency is heat to steam/total heat input through boiler. The efficiency is determined by how close the combustion is to stoichiometric over the whole firing range of the burner and the geometry of the boiler. As it is usually possible to set a gas burner up with less excess air than that one would set on any oil burner the heat take away in the flue tends to be less when firing on NG than MFO. So the efficiency can be higher on NG over the life of a burner than on MFO.

I also have to take issue with your statement above: Irrespective of what a poster said, no amount of setting up, fine tuning will change the CV of your fuel.

I have reread my post and nowhere can I find a statement which suggests that setting up would alter the CV of a fuel. The CV influences the amount of fuel required to release a particular amount of energy and that is what tofumaster is buying whether as NG or MFO or coconut shell. So in this case tofomaster's burner will have to burn NG at a higher mass flow rate, but equal heat release rate than when firing on MFO. Setting up influences the combustion efficiency. Two different things not to be confused.

To answer the question in the original post his burner will have to fire at the same heat release rate on NG as on MFO within the accuracy which it can be measured commercially. In old money you still need 970Btu to evaporate a pound of water f&@212C regardless of what the fuel is. The boiler case losses are going to be the same regardless of the fuel and the flue loss is also going to be very similar as argued earlier.

I used the word efficiency to illustrate to the OP he will use more gas than he did with MFO. If you use fuel with a lower Cv you will use more of it. We are actually talking about the fuel, not the boiler or its setup. His question was simply whether his installer told him the truth. We could have done some Cv calcs using the two fuels and still come to the same conclusion. Dont forget, one assumes that the installation and setup was done with mass balances and thus reply accordingly. You correctly asked whether this or that was done, however since he asked a specifc question, i assumed all was well and therefor i still believe my answer to on or very close to the button. For info, i had the same dilemma a number of years ago to change from HFO to LP gas. In my case the Cv of LP is higher than HFO, and therefor more attractive financially as well as environmentally. However, the cost of the gas, the storage requirement and the relevant safety requirements chased the cost too high. ( Bear in mind though that we did not, and still dont, have piped gas to our site.) I cjanged over to LFO instead. In my case the Cv was higher, and definitely the efficiency (steam raised with oil supplied and hence steam cost per ton of product) improved.